1. Field of the Invention
This invention relates generally to forming a sulfonated layer on an article, and more particularly to an apparatus and method for sulfonating an article and the articles made thereby.
2. Background Art
Exposure of the surfaces of plastic components to sulfur-containing compounds is known to enhance certain properties of these surfaces. This exposure is usually referred to as sulfonation. In one example, sulfonation of a plastic surface is known to form a barrier layer with decreased permeation to hydrocarbons and organic solvents. Such barrier layer formations are particularly useful in plastic gasoline tanks, filler necks, gasoline caps, and rollover valves.
In one such prior art sulfonation method, a sulfur-containing gas is contacted with the inner surface of a gasoline tank, thereby creating a sulfonation barrier layer to prevent permeation of gasoline. In this prior art process, the gasoline tanks are made of a sulfonatable polymer, i.e., one which has hydrogen atoms bonded to the carbon atoms replaceable by sulfonic acid groups. Examples of sulfonatable polymers include polypropylene, polyethylene, polybutylene, etc. Non-reactive polymers such as polytetrafluoroethylene are not sulfonatable.
The prior art sulfonation methods include processes in which sulfonatable polymers are treated with sulfur trioxide gas. In the prior art methods, the entire gasoline vessel and/or its complementary components are placed into a reaction chamber which is subsequently evacuated. Sulfur trioxide gas is then introduced into the chamber thereby contacting and sulfonating the articles within the chamber. Although this prior art method produces sulfonated articles with acceptable properties, the method tends to be slow, requiring the storage of unacceptably large volumes of sulfur trioxide.
U.S. Pat. No. 6,758,910 (the '910 patent) solves a number of problems of the prior art. For example, the '910 patent provides an apparatus for rapidly, inexpensively and efficiently sulfonating the surface of an article with a sulfonating gas. The methodology of the '910 patent allows for the formation of treated surfaces that are impermeable to hydrocarbons and organic solvents. However, the methods of the '910 patent tend to produce articles with a lingering acid content that lead to failure and degradation of final products that incorporate the sulfonated articles. Moreover, the sulfur monitoring methods disclosed in the '910 patent have not been found to be useful in that such monitoring is expensive and cumbersome to implement. These sulfur monitoring methods attempt to monitor the amount of sulfur in the reagents and not in the sulfonated articles. Accordingly, these methods are also somewhat unreliable.
Accordingly, there exists a need for improved methods of sulfonating plastic articles with reduced lingering acidity, and for a sulfonation process in which the amount of sulfonation in sulfonated articles is accurately measured.